The present invention relates to the design and structure of electronic devices in which transition of rare-gas and halogen atoms are electrostatically induced in solid thin films. The electric double layer and large polarization consequently formed in the films are utilized as the operation principle of the devices. The invention also relates to devices with electrodes and the processes for producing the devices.
Dielectricity and polarization of conventional dielectric materials used for electronic devices are not large enough for the future development of ultrahigh density memories, ultrahigh sensitivity sensors and a new generation of piezoelectric and actuator devices. Therefore, new materials that possess superb dielectric characteristics are required and have been investigated to replace conventional dielectric materials. Studies on the design and preparation of devices are also needed for the application of the new materials.
For example, BaTiO3 is widely known as a high-dielectric material and is used in many devices such as condensers and positive thermal coefficient (PTC) devices. However, the origin and mechanism of dielectric field formation in BaTiO3 crystals are due to the small dislocation, approximately 0.1 angstrom (0.1xc3x9710xe2x88x9210 m), of central Ti ions relative to the Ba ions in a unit cell.
Similarly, the relative dislocations of central ions in unit cells of conventional ferroelectric materials, such as PbTiO3, PbZrTiO3 and Bi-containing layer-by-layer compounds (so-called Y1 materials), are very small. Since these conventional crystalline materials have the essential restriction that the central ions must move within the sharply bound potential of crystalline unit cells, a long-range charge dislocation, such as a charge-separation on the order of three to four angstroms, cannot principally be expected. Therefore, these materials will not become superdielectric materials that exceed dielectric constants of conventional materials.
Some organic compounds such as liquid crystals also show spontaneous polarization. However, a dislocation of plus and minus charges originating from the presence of functional groups in these materials is limited inside the molecules. Therefore a long-range, i.e., three to four angstroms, charge-separation cannot be expected.
As is mentioned above, the advent of new materials that have charge-separation or dielectric constants one or two orders of magnitude larger than conventional materials have been eagerly awaited. If such materials were invented, they would be called superdielectric materials. The materials will show giant dielectricity and polarization.
On the other hand, excimer-formation is well known as a reaction resulting in a long-distance charge-separation. Excimer lasers and lamps have been commercialized using this reaction. Excimers are metastable species that are formed via high-energy input such as pulse-discharge or ultraviolet (UV) light irradiation on rare-gas/rare-gas or rare-gas/halogen mixtures. The possibility of applying the reaction to laser oscillation and UV light sources has already been realized and discussed in the 1960s.
UV lasers were then commercialized in the 1970s. At present, they are commonly used as light sources in stepper systems for semiconductor processes. Furthermore, successful development of excimer lamps that provide deep UV light has widened the application of excimer-formation reactions in the industry.
Both the physical and chemical understanding of the properties and characteristics of excimers have rapidly progressed. Energy potentials, formation and emission mechanisms, non-radiative relaxation processes and application of the excimer reactions to optical devices are well summarized and published in literature such as xe2x80x9cExcimer Lasers, Second Enlargement Edition, ed. by Ch. K. Rhodes, Springer-Verlag (Berlin, Heidelberg, New York, Tokyo, 1984).xe2x80x9d
However, application of excimer reactions to industrial fields other than optical devices has not been pursued to date; neither any research nor idea relating to the application of excimer reactions to electronic devices has been reported.
The inventor has long been engaged in researches on xe2x80x9cFormation and relaxation mechanisms of excimers in condensed mediaxe2x80x9d and xe2x80x9cPreparation and evaluation of electronic thin films.xe2x80x9d By combining knowledge and experience obtained through these researches, the inventor has formulated the following idea: If excimers could be formed in solid thin films under electrically static conditions, we will have devices that show huge polarization and an electric double layer.
First of all, the inventor""s views on characteristics of excimers as potential electronic devices are summarized below:
1) Huge Polarization
As is described before, the relative charge-separation or charge dislocation of conventional dielectric materials is on the order of 0.1 angstrom. On the other hand, excimers are formed when rare-gases and halogens are excited into full-charge-transfer states: plus and minus charges are separated in the excimer molecule by three to four angstroms. Hence excimers have huge polarization compared with conventional dielectric materials. If excimers could be formed in solid thin films, we would have superdielectric materials that have dielectricity two orders of magnitudes higher than conventional materials. Then these films can be applied to ultrahigh-density memories, ultrahigh-sensitivity sensors and a new generation of piezoelectric devices and actuators.
2) Stability in Solid
Excimers easily relax to their ground states by colliding with other molecules in the vapor phase and show low quantum efficiency. In contrast, all of the excimers formed in the solid phase can be used as the driving force of devices because of the lack of collisional deactivation processes.
3) Delocalization of Holes
Holes of excimers trapped in low-temperature solid Xe can be delocalized in the Xe matrix in a manner similar to electrons transiting into their Rydberg states. Delocalized holes are trapped in the matrix without recombining with electrons. Only when the temperature of the matrix is raised, does the recombination of holes and electrons proceed, and recombination luminescence, i.e., thermo-luminescence, is observed. This mechanism was first reported by N. Schwentner, M. E Fajardo and V. A. Apkarian in xe2x80x9cRydberg series of charge-transfer excitations in halogen-doped rare gas crystals,xe2x80x9d Chem. Phys. Lett., Vol 154, pp.237, (1989). If such delocalization process could be attained in a solod thin film following charge-separation reactions in excimer molecules, it would be possible to form and stabilize the electric double layer. This kind of device could give rise to ultrahigh capacitors, new types of limiters and memories.
4) Varieties of Combinations
There are many excimer species. Molecular sizes, energy levels in excitation and relaxation processes, and transition probabilities of excimers can be controlled by changing the combination of parent atoms, i.e., rare-gases (Ar, Kr, Xe) and halogens (F, Cl, Br, I). Therefore, we can choose the rare-gas and halogen dopants in accordance with the structure and band gap of the host matrix. The flexibility in combining rare-gases and halogens widens the freedom in designing devices. In particular, Xe is very useful for device design, since it can react and make charge-transfer complexes with many kinds of atoms and radicals other than halogens, such as sulfur (S), oxygen (O) and hydroxide (OH) radicals.
Note that all of the characteristics and properties of excimers mentioned above have been investigated and observed in cryogenic rare-gas crystals and liquids by optically exciting the systems with UV light or by electric discharge. In order to apply these characteristics and properties to electronic devices in the real world, development of the following new technologies has been indispensable: doping of parent atoms of excimers into electronic materials such as ceramics; and excimer-formation under ambient temperature and pressure by a static electrical field. The concrete technological problems and strategies of the inventor to develop these breakthroughs are summarized in a) to d).
a) Selection of Host Materials
Since crystals have little flexibility in their matrices, possible dopants for them are very limited. Therefore, the inventor thought it important to select amorphous materials as hosts for parent atoms of excimers. Amorphous materials have an advantage in that they can contain various substances. For example, organic dyes that have high molecular weights can be embedded in a flexible network of amorphous SiOx. Such amorphous compounds should be suitable hosts for doping rare-gases and halogens that have large atomic diameters, and for charge-transfer reactions between them.
b) Doping of Parent Atoms of Excimers
Excimers are gas phase molecules at ambient temperatures and pressures. Therefore, it is difficult to confine them in solids and to operate them under ambient circumstances. The inventor has succeeded in doping parent atoms of excimers by sputtering process that is one representative of a non-equilibrium processes. During the formation of host-matrix thin films by sputtering process, parent atoms of excimers are used as sputtering gases. The parent atoms are ionized and form high-energy plasmas in the process. These high-energy species can successfully enter and be trapped in thin films.
c) Operation at Low Voltages
High-energy excitation processes, such as electric discharge or UV light irradiation, are required to produce excimers, since energy gaps between the ground and excited states of excimers are very wide. However, it is difficult to apply electrical discharge or UV light irradiation to solid devices doped with parent atoms of excimers, since the system becomes complicated, expensive and massive. Operation of the systems under one to several tens of DC or AC volts is a precondition for practical use of them as electronic devices. If parent atoms of excimers were doped in bulk materials, it would be impossible to induce charge-transfer reactions under such low electric fields. Therefore, the inventor pursued thin film fabrication, and developed a breakthrough in operating the system under low voltages. The alternative application of excimer devices by light irradiation will be discussed later in this patent.
d) Operation Temperature
The delocalization of hole of excimers has only been observed in cryogenic Xe matrices. The inventor at first thought it would be necessary to decrease the operating temperature to exploit the delocalization characteristics of excimers in electronic devices. That is, cooling the device seemed to be important to separate and stabilize the plus and minus charges.
However, astonishingly enough, when the device was prepared and tested, it was found that charge-transfer reactions can be reversibly repeated in the film, and the device showed large polarization and space charge at room temperature. No cooling was necessary for the operation.
The inventor has conducted researches to develop fundamental breakthroughs in the four problems mentioned above, and has completed the establishment of new technologies. They are the body of this invention. As a result, many types of devices have been created using these technologies. These devices, hereafter, will be called xe2x80x9csolid-state excimer devicesxe2x80x9d as a general term.
The invention realizes useful electronic devices that utilize the huge polarization and electric double layer generated by excimer formation in solid thin host films doped with excimer forming atoms (rare-gases and halogens: hereafter the parent atoms of excimers) at ambient temperature and pressure.
The fundamental scheme of the invention is the single layer solid-state excimer device. This device consists of a thin film fabricated by selecting a material from insulating metal oxides, at least one dopant from the parent rare-gas atoms (Ar, Kr and Xe) of excimers, and at least one dopant from parent halogen atoms (F, Cl, Br and I) of excimers. The dopants can be doped in the thin film during its fabrication process.
In other words, the thin film doped with the parent atoms of excimers is the most important and essential constituent element of the invention. Upper and lower electrodes can be added to the film when they are needed, and practical devices can be obtained by fabricating the electrodes and thin film together on a substrate. The details of the structure and the preparation process of the device are explained in the following sections.
Any kind of material can be used as a substrate of the device. For example, silicon, germanium, gallium arsenide, indium phosphorus, silicon oxide, aluminum oxide, metal plates and foils, glasses and plastics, can be selected as substrate materials. Neither precise orientation nor surface smoothness is required for the substrates. The preparation of the lower electrode, which is usually inserted between the substrate and the thin film, is unnecessary if conductive materials such as metal plates and foils are used as substrates, or if the film is excited by methods other than applying voltage to the film. When a film fabricated on an insulating substrate is to be excited by applying voltage, a lower electrode is needed between the substrates and films. In this case, any kind of conductive material, such as gold, silver, platinum, indium tin oxide (ITO) and iridium oxide, can be used as a material for the lower electrodes. The lower electrode does not require specific characteristics other than conductivity, and neither precise orientation nor surface smoothness is required for the electrode.
The reason why neither crystallinity nor surface precision is required for substrates and electrodes is that the films doped with parent atoms of excimers can be fabricated on any kind of surface, since the films are amorphous. If reaction or diffusion among the films, electrodes and substrates is anticipated, reaction-preventive thin films (buffer layers) represented by such materials as cerium oxide, aluminum oxide and yttrium-stabilized zirconium oxide (YSZ) can be inserted between substrates and electrodes or between electrodes and the films.
Doping of parent atoms of excimers into host materials can be achieved using a sputtering process where sputtering gases that contain the parent atoms are used during the fabrication of host materials on substrates or on the lower electrodes prepared on substrates.
In this procedure, the selected parent atoms of excimers are trapped inside thin films as a target made of a selected host material for the film is sputtered by gas-phase discharge in the sputtering gases that contain the parent atoms. The film is fabricated together with the dopant parent atoms of excimers on a substrate or on a lower electrode prepared on the substrate. For the sputtering gas, at least one kind of atoms should be selected from the rare-gases, Ar, Kr and Xe, and at least one kind of atom should be selected from the halogens, F, Cl, Br and I. Other kinds of gases such as Ne, He or N2 can simultaneously be used as remaining component of the sputtering gas, but they are not indispensable for the preparation of the devices. The sputtering conditions and dose of the parent atoms of excimers can be controlled by adjusting the ratio and flow rates of rare-gas/halogen mixtures as well as the sputtering pressure.
The single layer solid-state excimer devices are to be used as ferroelectric devices as described in the following paragraphs and example-1. Insulating host materials are favored for the thin films in these devices, since current leakage is prohibited in ferroelectric applications. For example, either crystalline or amorphous phases of the following materials, as well as their solid solutions can be used as the host for single layer solid-state excimer devices: aluminum oxide, barium oxide, bismuth oxide, cerium oxide, cobalt oxide, copper oxide, iron oxide, gallium oxide, gadolinium oxide, germanium oxide, lanthanum oxide, lithium oxide, magnesium oxide, manganese oxide, molybdenum oxide, niobium oxide, neodymium, oxide, nickel oxide, lead oxide, silicon oxide, strontium oxide, titanium oxide, vanadium oxide, tungsten oxide, yttrium oxide and zirconium oxide.
In the state of targets, the morphologies of host materials can either be crystalline or amorphous phases. However, during the sputtering stage the target materials should be fabricated to be amorphous or nanocrystalline thin films so that parent atoms with large atomic diameters can be trapped into the host films. In amorphous phases, the compositions of materials sometimes deviate from their stoichiometric values. For example, the composition of a silicon oxide host, i e., the O/Si ratio, produced by the procedure described above was 1.9, while the stoichiometric composition of the target quartz is 2. Therefore, the composition of such host films should be written as SiOx.
In this section the preparation process of thin films doped with parent atoms of excimers is explained in detail. There are many conventional processes for the fabrication of thin films. Sputtering, plasma chemical vapor deposition (plasma CVD) and ion implantation are well-known processes for the fabrication, and any of them can be utilized for the purpose of fabricating a thin film doped with parent atoms of excimers as far as doping of rare-gases and halogens into films is possible. However, sputtering is superior to the other processes in terms of simplicity, cost of operation and ease of preparing lower and upper electrodes. A brief comparison of the representative three fabrication processes is given below.
i) Sputtering
By mixing parent atoms of excimers in sputtering gas, these atoms can easily be doped into host films by sputtering. Furthermore, it is possible to carry out the entire fabrication processes using one apparatus, if multi-target sputtering equipment is used. That is, fabrication of a lower electrode on a substrate, a thin film doped with parent atoms of excimers on the lower electrode, and an upper electrode on the thin film can be carried out in the same equipment. There is no need to convey a substrate in and out of the chamber, fit a substrate to the substrate holder and evacuate the chamber three or four times, if a mask-patterning process is not necessary. Therefore, products with few stains and scars are obtainable.
ii) Plasma CVD
It is possible to dope parent atoms of excimers in thin films by plasma CVD, since rare-gas and halogens are ionized to form plasmas which have sufficiently high-energy levels for the atoms to be embedded into the films. However, the doping process and procedure in plasma CVD is more complicated than those in sputtering.
For example, let us think about one of the simplest plasma CVD processes in which SiOx thin films are fabricated from silane gas and parent atoms of excimers are doped simultaneously. We have to use many gases in this process: silane gas should be used as the raw material for the host film, oxygen gas is also needed as oxidant for silane, rare-gas and halogen gas should be added to these gases as dopants. Therefore, the apparatus and procedure become very complicated. In addition to the doped rare-gases and halogens, impurities such as hydrogen and hydroxyl radicals could also be trapped in the films, and these impurities will degrade the film properties. Furthermore, it is difficult to fabricate lower and upper electrodes using the same apparatus as is used for fabrication of the SiOx films. Therefore, the plasma CVD does not have any advantages over the sputtering.
iii) Ion Implantation
Embedding many kinds of ions into thin films and substrates is widely performed by ion implantation. It is also possible to dope rare-gas and halogen ions into films by ion implantation. However, the apparatus that enables simultaneous and uniform implantation of plus (rare-gas) and minus (halogen) ions into a film or that enables fabrication and implantation procedures simultaneously is very complicated and expensive. Such an apparatus is very rare and limited in use to date. Moreover, the operating cost of such apparatus is very high. Therefore, ion implantation is inferior to sputtering for fabricating films doped with parent atoms of excimers.
As is explained above, these processes can be applied for fabricating the films doped with parent atoms of excimers, and the films can be fabricated either directly on a substrate or on a lower electrode formed on a substrate. An upper electrode using any kind of material can be formed on the films to apply voltage between the lower and upper electrodes. Any kind of conductive material, such as gold, silver, platinum, indium tin oxide (ITO) and iridium oxide, can be used as a material for the upper electrodes, and the upper electrodes do not require orientation, epitaxiality or surface precision.
Note that the use of lower and upper electrodes is not described in claims No. 1 and No. 3 of the invention, because the possible application of solid-state excimer devices to systems that do not need electrodes is considered. However, the invention does not deny the use of the electrodes.
The action of solid-state excimer devices is explained by a fundamental example, in which a SiOx film is used as an insulating host material and both Xe and F atoms are codoped as parent atoms of XeF excimers into the SiOx film. This structure is defined and hereafter called as single layer solid-state excimer devices.
In this case, monovalent F atoms doped in a SiOx film replace divalent oxygen atoms. Therefore, dangling bonds exist adjacent to F atoms. Xe atoms are trapped in the film partially supplying their outer-shell electrons to the dangling bonds. Therefore Xe atoms exist in the neighborhood of F atoms in the film. When several or some tens of volts is applied to the film, a large electric field on the order of several tens of kV/cm is generated in the film, since the film is very thin. Because of the large electric field, electrons of doped Xe atoms hop toward F atoms in the direction of the electric field, and excimers are formed by the following charge-transfer reaction (harpooning reaction):
Xe+Fxe2x86x92Xe+F
If Xe and F atoms are codoped in the same insulating thin film, outer-shell electrons of Xe atoms compensate for the dangling bonds in the film in their ground state, and an ideal ferroelectric film with very little current leakage can be obtained. Solid-state excimer devices using such thin films can be used as capacitors, piezoelectric devices, ultrasonic devices, actuators, pyroelectric sensors and ferroelectric memories.